Semiconductor device fabrication involves various processing steps which may fall into four general categories: deposition, removal, patterning, and modification of electrical properties. Deposition is any process that grows, coats, or otherwise transfers a material onto the wafer. Available technologies include physical vapor deposition (PVD), chemical vapor deposition (CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE) and more recently, atomic layer deposition (ALD) among others. Removal is any process that removes material from the wafer; examples include etch processes (either wet or dry) and chemical-mechanical planarization (CMP). Patterning is the shaping or altering of deposited materials, and is generally referred to as lithography. For example, in conventional lithography, the wafer is coated with a chemical called a photoresist; then, a machine called a stepper focuses, aligns, and moves a mask, exposing select portions of the wafer below to short wavelength light; the exposed regions are washed away by a developer solution. After etching or other processing, the remaining photoresist is removed by plasma ashing. Modification of electrical properties has historically entailed doping transistor sources and drains (originally by diffusion furnaces and later by ion implantation). These doping processes are followed by furnace annealing or, in advanced devices, by rapid thermal annealing (RTA); annealing serves to activate the implanted dopants. Modification of electrical properties now also extends to the reduction of a material's dielectric constant in low-k insulators via exposure to ultraviolet light in UV processing (UVP). Modern chips have up to eleven metal levels produced in over 300 sequenced processing steps.
Many toxic materials are used in the fabrication process. These include: poisonous elemental dopants, such as arsenic, antimony, and phosphorus, poisonous compounds, such as arsine, phosphine, and silane, and highly reactive liquids, such as hydrogen peroxide, fuming nitric acid, sulfuric acid, and hydrofluoric acid.
One chemical that has recently been introduced into the process is ozone (O3). DNS (Dainippon Screen Manufacturing Company) Single wafer processing wet tools DC-08, DC-09, DC-10 & DC-11 use a new cleaning chemistry of sulfuric acid and Ozone.
Ozone (O3) is a form of oxygen that consists of three oxygen atoms joined together into a molecule. This form of oxygen has significantly different characteristics than the common oxygen molecule (O2), which consists of two oxygen atoms. The ordinary O2 form of oxygen is, of course, present in the air we breathe and is indeed necessary for life.
The role of ozone in the environment is more complicated. First, ozone in the upper atmosphere plays an important role in protecting life on earth by absorbing dangerous short wavelength UV from the sun. However, it is harmful in the lower atmosphere since it is an irritant when breathed and is therefore an undesirable air pollution component.
The ozone used in fabrication must be abated before being exhausted into the environment. Conventional methods of ozone abatement utilize carbon filters to reduce the ozone and sulfuric acid to acceptable levels. The final stage of abatement systems is an acid scrubber to remove the remaining sulfuric acid. The acid scrubber is a water wash that the exhaust is passed through. One issue with the carbon filters is a result of the need to constantly change out the filters over time. This requires significant maintenance and cost.